The present invention relates to a method for polymerising olefins, a catalyst for such a polymerisation and a method for producing such a catalyst. A particular aspect of the present invention relates to a method for producing linear low density copolymers of ethylene, hereinafter referred to as “LLDPE” and high density polymers, hereinafter referred to as “HDPE”.
The present invention is particularly suitable for the co-polymerisation of olefins in the gas phase. Such gas phase polymerisation processes can be conducted for example by introducing the gaseous monomer and comonomer into a stirred and/or gas fluidised bed comprising polyolefin and a catalyst for the polymerisation.
In the gas fluidised bed polymerisation of olefins, the polymerisation is conducted in a fluidised bed reactor wherein a bed of polymer particles is maintained in a fluidised state by means of an ascending gas stream comprising the gaseous reaction monomer. The start-up of such a polymerisation generally employs a bed of polymer particles similar to the polymer which it is desired to manufacture. During the course of polymerisation, fresh polymer is generated by the catalytic polymerisation of the monomer, and polymer product is withdrawn to maintain the bed at more or less constant volume. An industrially favoured process employs a fluidisation grid to distribute the fluidising gas to the bed, and to act as a support for the bed when the supply of gas is cut off. The polymer produced is generally withdrawn from the reactor via a discharge conduit arranged in the lower portion of the reactor, near the fluidisation grid. The fluidised bed consists in a bed of growing polymer particles. This bed is maintained in a fluidised condition by the continuous upward flow from the base of the reactor of a fluidising gas.
The polymerisation of olefins is an exothermic reaction and it is therefore necessary to provide means to cool the bed to remove the heat of polymerisation. In the absence of such cooling the bed would increase in temperature and, for example, the catalyst becomes inactive or the bed commences to fuse. In the fluidised bed polymerisation of olefins, the preferred method for removing the heat of polymerisation is by supplying to the polymerisation reactor a gas, the fluidising gas, which is at a temperature lower than the desired polymerisation temperature, passing the gas through the fluidised bed to conduct away the heat of polymerisation, removing the gas from the reactor and cooling it by passage through an external heat exchanger, and recycling it to the bed. The temperature of the recycle gas can be adjusted in the heat exchanger to maintain the fluidised bed at the desired polymerisation temperature. In this method of polymerising alpha olefins, the recycle gas generally comprises the monomer and comonomer olefins, optionally together with, for example, an inert diluent gas such as nitrogen or a gaseous chain transfer agent such as hydrogen. Thus, the recycle gas serves to supply the monomer to the bed, to fluidise the bed, and to maintain the bed at the desired temperature. Monomers consumed by the polymerisation reaction are normally replaced by adding make up gas or liquid to the polymerisation zone or reaction loop.
Linear low density polyethylene polymers possess properties which distinguish them from other polyethylene polymers such as high density polymers, including homopolymers of polyethylene. Certain of these properties are described in the Anderson et al U.S. Pat. No. 4,076,698.
When the polyethylene resins are fabricated into products, it is imperative to control the molecular weight distribution of the resins since, as is known to those skilled in the art, the properties of the products can be predicted from the molecular weight distribution of the resins.
One of the measures of the molecular weight distribution of the resin is melt flow ratio (MFR), which is the ratio of high load melt index (HLMI or I21) to melt index (MI or I2) for a given resin. MFR is defined herein as the ratio of the high load melt index (HLMI or I21) divided by the melt index (MI or I2). The melt flow ratio is believed to be an indication of the molecular weight distribution of the polymer, the higher the value, the broader the molecular weight distribution. Resins having relatively low MFR values, e.g., of about 20 to about 45, preferably 20 to 35, have relatively narrow molecular weight distributions. Additionally, LLDPE resins having such relatively low MFR values produce films of better strength properties than resins with high MFR values.
For higher density polyethylene the n value can be used as a measure of the molecular weight distribution of the polymer. This n value is calculated as [log10(I8.5/I0.325)]/[log10(8.5/0.325)] where I8.5 and I0.325 are the melt indexes measured under 8.5 Kg and 0.325 Kg respectively. The higher the n value, the broader the polymer molecular weight distribution.
The molecular weight of the ethylene (co)polymers may be controlled in a known manner, e.g., by using hydrogen. With the catalysts produced according to the present invention, molecular weight may be suitably controlled with hydrogen when the polymerisation is carried out at relatively low temperatures, e.g., from about 30° C. to about 115° C. This control of molecular weight may be evidenced by a measurable positive change in melt index (I2) of the polymer produced. For HDPE production it is essential that the catalyst used has a sufficiently high response to hydrogen since hydrogen will decrease the catalyst activity. It is therefore important that such catalysts produce polymers with the required molecular weight, using the minimum of hydrogen.
It is therefore a primary object of the present invention to provide a high activity catalyst for the polymerisation of olefins yielding LLDPE products of a relatively narrow molecular weight distribution. It is an additional object of the present invention to provide a catalytic process for polymerising olefins which yields polyethylene of a relatively narrow molecular weight distribution at high productivity.
It is also a primary object of the present invention to provide a high activity catalyst for the polymerisation of olefins yielding HDPE products of the required molecular weight distribution. It is an additional object of the present invention to provide a catalytic process for polymerising olefins which yields polyethylene of a required molecular weight distribution at high productivity.
It is a further object of the present invention to provide a high activity catalyst for the polymerisation of olefins which yields polyethylene of narrow molecular weight distributions, high melt indices and exceptionally low weight swell. Such resins are particularly useful for injection moulding and rotational moulding applications.